Injection mold with surface coating of the inner surface

Abstract

A blow mold for PET bottles has the inner surface which contacts the hot PET during the blowing operation, coated with a layer of ceramic material having a nanometric thickness, of which at least one thickness layer is AI.sub.2O.sub.3 and/or TiO2 deposited by ALD technique. Due to this technique, the PET preforms are subject to less friction during the contact with the interior of the mold.

Claims

1. A mold having steel walls for the production of preforms made of polyethylene terephthalate, PET, by injection molding, comprising a coating of ceramic material with a thickness between 90 and 120 nm which coats the walls of the mold, wherein the coating is made of either a layer of Al.sub.2O.sub.3 or a multilayer structure having a first layer made of Al.sub.2O.sub.3 and a second layer made of TiO.sub.2, the mold producing the preforms with L/t>50, when L>100 mm or preforms with L/t>45 when L<100 mm, where “L” and “t” are the total length of the preform and the wall thickness of the preform, respectively.

2. A process for obtaining molds according to claim 1, having a reduced chemical interaction with, the PET, such a process comprising: a step of cleaning the surface of the mold walls, a step of polishing the mold walls, a step of depositing on the mold walls a ceramic coating of thickness in the range between 90 and 120 nm made of either a layer of Al.sub.2O.sub.3 or a multilayer structure having a first layer made of Al.sub.2O.sub.3 and a second layer made of TiO.sub.2.

3. A process according to claim 2, wherein the Atomic Layer Deposition, ALD, technique is provided for depositing the ceramic coating.

4. Use of a mold according to of claim 1 for the production of preforms made of PET, with L/t>50, when L>100 mm or preforms with L/t>45 when L<−100 mm, were “L” and “t” are the total length of the preform and the wall thickness of the preform, respectively.

Description

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

(1) The application of a special coating using the ALD technology, but also other technologies such as PVD or CVD alone or in combination, is carried out with the aim of reducing the chemical-physical interaction between the mass of molten PET and the steel molding surfaces, or walls, of a mold for injection molding. A reduction of the chemical-physical interactions and therefore of the molecular adhesion, between the mass of PET and the steel, means a reduction in the friction between PET and steel and therefore greater ease of injection with reduced danger of “short shots”. Among the most chemically inert materials used for coating molding surfaces are thin ceramic based, nanocomposite coatings, such as Al.sub.2O.sub.3, TiN, TiO.sub.2 or TiAlN or CrN or CrN and AlN, etc. These nanostructured coatings, in addition to reducing the chemical-physical interactions reducing the friction coefficient between the molten material and the molding surfaces, have been shown to be superior to standard coatings for certain qualities, such as degree of homogeneity, hardness, resistance to fracture, wear, corrosion, producing also a better finish of the treated surfaces. The cleaning of the overlapping molding surfaces prior to deposition is a critical element of the process. The polishing of the surfaces is carried out in order to obtain already a lower friction coefficient, also in anticipation of the subsequent process of detachment of the preform from the die and also for the preparation of the surface to the deposition process. Once the polishing has been carried out, it is possible to carry out the deposition process of the nanolayer or nanolayers of ceramic nanomaterial. The adhesion of the deposition, and its durability are also critical elements since it is provided that the deposition should remain adherent to the steel surface for several million cycles of production of preforms. A proper balance must therefore be found between the thickness of the layer or layers, which concerns the duration of the deposition even in case of wear and tear over time, and the mechanical strength which is greater for a thin layer. These elements also depend on the material used for the deposition and the use of double layers of different material can offer greater flexibility in the choices, it being possible to vary the relative thickness of the single layers, as well as their composition.

Examples

(2) Further advantages of the invention are apparent from the following table in which there are examples of molds, provided by way of non limiting example.

(3) TABLE-US-00001 MATERIAL OF THE TOTAL AVERAGE FRICTION MOLDING DEPOSITION FRICTION REDUCTION SURFACE THICKNESS COEFFICIENT PERCENTAGE polished steel — 0.20 — Single 100 nm (nominal) 0.14 30% nanolayer of 92 nm (actual).sup.  Al.sub.2O.sub.3 Complex 100 nm (nominal) 0.13 35% nanolayer - 110 nm (actual) .sup.  two layers: one of Al.sub.2O.sub.3 and one of TiO.sub.2